Usually, an electromagnetic fuel injector comprises a tubular supporting body displaying a central channel, which serves the fuel feeding function and ends with an injection nozzle adjusted by an injection valve controlled by an electromagnetic actuator. The injection valve is provided with a shutter, usually named “needle”, which is firmly connected to a movable anchor of the electromagnetic actuator to be displaced between a closing position and an opening position of the injection nozzle against the bias of a spring which tends to keep the shutter in the closing position.
An example of electromagnetic fuel injector of the above-described type is provided by U.S. Pat. No. 6,027,050, which relates to a fuel injector provided with a shutter which, at one end, cooperates with an internal seat of the injection valve and, at the opposite end, is integral with a movable anchor of an electromagnetic actuator; the shutter is guided at the top by the anchor and at the bottom by a guide obtained along the internal seat of the injection valve.
The known electromagnetic injectors of the above-described type are very common because they combine good performances and low costs. However, such injectors with electromagnetic actuation of the shutter are not able to operate at relatively high fuel pressures; for this reason, injectors with hydraulic actuation of the shutter have been proposed, i.e. injectors in which the displacement of the shutter from a closing position to an opening position against the bias of the previously mentioned spring no longer occurs against the direct bias of the electromagnetic actuator, but occurs under the bias of hydraulically originated forces controlled by the electromagnetic actuator, which no longer serves the function of power member, but functions as a control member. An example of injector with hydraulic actuation of the shutter is provided by EP-A-1036932, by EP-A-0921302 and by WO-A-0129395.
Specifically, in an injector with hydraulic actuation of the shutter, the fuel which enters the injector comes from a high-pressure pump; a considerable amount of this fuel, which is aspirated from a tank, is not however involved in the combustion process inside the cylinder, and is returned to the tank itself. Indeed, of all the fuel fed to the injector, a first fraction reaches the injection valve through the central feeding channel, while a second fraction fills a control chamber arranged upstream of the shutter and serving as a chamber of a hydraulic cylinder, a piston of which is directly connected to the shutter. The hydraulic cylinder displays an exhaust connected to the tank by means of a fuel return pipe and controlled by a sealing member controlled by the electromagnetic actuator. When the magnet of the electromagnetic actuator is energized, the sealing member is displaced by connecting the chamber of the hydraulic cylinder to the return pipe so as to determine a pressure drop inside the hydraulic cylinder and to allow the shutter to be displaced to the open position.
An injector with hydraulic actuation of the shutter displays good dynamic performances and is able to operate at relatively high fuel pressures, but is complex and relatively expensive because it requires to make an internal hydraulic circuit controlled by an electromagnetic or, alternatively, a piezoelectric actuator. Furthermore, using an injector with hydraulic actuation of the shutter always provides a return flow of fuel at ambient pressure to the tank. This return flow represents a loss of energy and tends to heat the fuel within the tank. Finally, the high-pressure pump must also be over-dimensioned with respect to the actual fuel consumption by the engine, because part of the pumped fuel is not injected into the cylinders, but is re-introduced at ambient pressure into the tank; i.e. the high-pressure pump must supply both the fuel used by the engine and the fuel required for the operation of the injectors with hydraulic actuation of the shutter.